Magnetic resonance imaging apparatuses (hereinafter, “MRI apparatuses”, as necessary) are apparatuses used for imaging information on the inside of a subject, by using a nuclear magnetic resonance phenomenon. An MRI apparatus acquires data called k-space data, by sampling nuclear magnetic resonance signals from specific atoms (e.g., hydrogen atoms) that are present on the inside of an object, by using coils. Further, the MRI apparatus reconstructs a Magnetic Resonance image (hereinafter, an “MR image”, as necessary) by applying a Fourier transform to the k-space data.
The nuclear magnetic resonance signals (hereinafter, “MR signals”, as necessary) are sampled as one-dimensional data. Thus, to obtain two- or three-dimensional reconstructed images, the MRI apparatus acquires the k-space data required by the reconstructing process by repeatedly performing one-dimensional sampling processes in the k-space. By sampling (full sampling) the k-space data at the same resolution as that of the MR image, it is possible to reconstruct the image by applying the Fourier transform to the obtained k-space data.
It is known that the sampling process of an MRI apparatus takes a long time. Further, the reconstructing process using the Fourier transform is based on the assumption that the subject remains still. If the subject has a movement during the imaging process, the k-space data obtained from the sampling process turns out to be data obtained by taking images of the subject that change from one imaging time to another. For this reason, if the Fourier transform is applied to such k-space data so as to reconstruct one image, there is a possibility that significant artifacts may occur.
As a method for reducing artifacts caused by movements, a method called a Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction (PROPELLER) method is known. According to the PROPELLER method, an MRI apparatus acquires a number of lines called blades, in such a manner that the blades pass the center of a k-space. Further, while varying the sampling direction (the read-out direction), the MRI apparatus repeatedly acquires blades a number of times. Subsequently, after having acquired k-space data, the MRI apparatus corrects movements among the blades and integrates all the blades together, before performing a reconstructing process. According to this method, although the imaging time period increases, it is possible to reduce artifacts caused by movements. Further, even if another device different from the MRI apparatus takes images of the subject at the same time, it is possible to reduce artifacts caused by movements, by detecting the movement with the use of the taken images.